GRAIL Satellites To Enter Orbit Around Moon This Weekend

December 31, 2011

NASA’s twin GRAIL satellites are expected to enter tandem orbits around the moon this weekend, beginning their mission to map its gravitational field and solve some of its mysteries.

The first of the GRAIL (Gravity Recovery and Interior Laboratory) vehicles, GRAIL-A, is currently scheduled to be put into orbit starting at 4:21pm EST on December 31, according to the official mission website. The second, GRAIL-B, will follow suit at 5:05pm EST on New Year’s Day.

“Our team may not get to partake in a traditional New Year’s celebration, but I expect seeing our two spacecraft safely in lunar orbit should give us all the excitement and feeling of euphoria anyone in this line of work would ever need,” David Lehman, project manager for GRAIL from NASA’s Jet Propulsion Laboratory in Pasadena, California, said in a statement Wednesday.

According to BBC News Science Correspondent Jonathan Amos, the probes, which departed from Cape Canaveral, Florida last September are expected to provide scientists with a bevy of new insight into the internal structure of Earth’s satellite, and the information they collect could offer clues as to how the moon formed and why the appearance of its near and far sides are so different.

During the final approach sequence, NASA says that both orbiters will approach the moon from the south. GRAIL-A’s lunar orbit insertion burn will take approximately 40 minutes, and GRAIL-B’s, which will come 25 hours later, will last 39 minutes.

“The insertion maneuvers will place each orbiter into a near-polar, elliptical orbit with a period of 11.5 hours,” the American aeronautics agency said. “Over the following weeks, the GRAIL team will execute a series of burns with each spacecraft to reduce their orbital period from 11.5 hours down to just under two hours. At the start of the science phase in March 2012, the two GRAILs will be in a near-polar, near-circular orbit with an altitude of about 34 miles (55 kilometers).”
During the data collection phase, the two probes will transmit radio signals that determine the exact distance between each of them during their respective orbits.

They will alternately move closer to or father away from each other as they fly over zones of increased or reduced gravity, and the changes in relative velocity will be recorded by an instrument onboard both GRAIL crafts.

“Grail will improve our knowledge of the Moon’s nearside gravity by more than 100 times over what was previously known, and by more than 1,000 times over what was known on the far-side,” lead investigator Dr. Maria Zuber, of the Massachusetts Institute of Technology (MIT) researcher, told Amos on Friday.

“We believe the Moon formed from the impact of a Mars-sized object into Earth, but we understand little really of how this happened and how the [lunar body] cooled off after the violent event,” she added. “Given that we’ve sent so many missions that have studied the outside of the Moon, it seems that the answer is not on the surface. The answer is locked in the interior.”

The GRAIL satellites will collect data for a total of 82 days, until the moon enters an eclipse behind the Earth in early June, according to BBC News. Provided that their batteries are able to survive this event, they will most likely be sent on a second mapping cycle during the second half of 2012.